Question: Which of these numbers is composite? ${11,\ 21,\ 37,\ 47,\ 71}$
Explanation: A composite number is a number that has more than two factors (including 1 and itself). 11, 37, 47, and 71 each have only two factors. The factors of 21 are 1, 3, 7, and 21. Thus, 21 is the composite number.